The term "formose" in the context of the present invention, means the known mixtures of low molecular weight polyhydroxyl compounds (polyhydric alcohols, hydroxyaldehydes and hydroxyketones) which are obtained from the condensation of formaldehyde hydrate.
The preparation of mixtures of polyhydric alcohols, hydroxyaldehydes and hydroxyketones by the autocondensation of formaldehyde hydrate has been described in the literature, for example by Butlerow and Loew, in Annalen 120, 295 (1861) and J.pr.Chem. 33, 321 (1886); Pfeli, chemishce Berichte 84, 229 (1951); Pfeil and Schroth, chemische Berichte 85, 303 (1952); R. D. Partridge and A. H. Weiss, Carbohydrate Research 24, 29-44 (1972); the formoses of glyceraldehyde and dihydroxyacetone according to Emil Fischer, German Pat. Nos. 822,385; 830,951 and 884,794 and U.S. Pat. Nos. 2,224,910, 2,269,935 and 2,272,378 and British Pat. No. 513,708. These known processes however, have certain disadvantages such as toxicologically harmful catalysts, poor volume/time yields and colored by-products. New processes have recently been developed by which virtually colorless formoses which are free from unwanted by-products can be obtained in high yields with the aid of conventional catalysts.
According to one of these new processes, the condensation of formaldehyde hydrate is carried out in the presence of catalysts consisting of soluble or insoluble lead(II) salts or of lead(II) ions bound to high molecular weight carriers, and is the presence of a cocatalyst which consists of a mixture of hydroxyaldehydes and hydroxyketones such as can be obtained from the condensation of formaldehyde hydrate and which is characterized by the following molar ratios:
Compounds with 3 carbon atoms/compounds with 4 carbon atoms: 0.5:1 to 2.0:1 PA0 Compounds with 4 carbon atoms/compounds with 5 carbon atoms: 0.2:1 to 2.0:1 PA0 Compounds with 5 carbon atoms/compounds with 6 carbon atoms: 0.5:1 to 5.0:1.
The proportion of components having from 3 to 6 carbon atoms is at least 75% by weight, preferably more than 85% by weight, based on the total quantity of cocatalyst.
The reaction temperature used is generally from 70.degree. C. to 110.degree. C., preferably from 80.degree. C. to 100.degree. C. The pH of the reaction solution is initially adjusted to 6.0 to 8.0, preferably 6.5 to 7.0 by controlled addition of an inorganic or organic base until 10 to 60%, preferably 30 to 50% of the starting materials have been converted. Thereafter the pH is adjusted to 4.0 to 6.0, preferably 5.0 to 6.0. It was surprisingly found that by controlling the pH in this special manner and subsequently cooling at various different residual formaldehyde contents (0 to 10% by weight, preferably 0.5 to 6% by weight), the distribution of products in the polyol, hydroxyaldehyde and hydroxyketone mixtures could be varied in a reproducible manner.
When the autocondensation of formaldehyde hydrate has been stopped by cooling and/or by inactivation of the lead catalyst with acids, the catalyst may be removed in known manner and the water contained in the products evaporated off. Details of this procedure may be found in German Offenlegungsschrift No. 2,639,084.
Another possible method for preparing highly concentrated colorless formoses with high volume/time yields consists of condensing aqueous formalin solutions and/or paraformaldehyde dispersions in the presence of a soluble or insoluble metal catalyst and of a cocatalyst which has been prepared by partial oxidation of a dihydric or higher hydric alcohol containing at least two adjacent hydroxyl groups and having a molecular weight of between 62 and 242 or a mixture of such alcohols. The pH of the reaction solution is kept between 6.0 and 9.0 by controlled addition of a base until 5 to 40% conversion is obtained. Thereafter, the reaction solution pH is adjusted to between 4.5 and 8.0 until the condensation reaction is stopped. During this letter phase of the reaction the pH is 1.0 to 2.0 units lower than during the first phase of the reaction. The reaction is stopped at a residual formaldehyde content of 0 to 10% by weight by inactivating the catalyst. The catalyst is then removed. This method has been described in detail in German Offenlegungsschrift No. 2,718,084.
High quality formoses can also be obtained by the condensation of formaldehyde in the presence of a metal catalyst and more than 10% by weight, based on formaldehyde, of one or more di- or poly-hydric low molecular weight alcohols and/or higher molecular weight polyhydroxyl compounds. Formosepolyol mixtures of this kind are described in German Offenlegungsschrift No. 2,714,104.
It is particularly economical to prepare formose directly from formaldehyde-containing synthesis gases, i.e. without first preparing aqueous formalin solutions or paraformaldehyde. In order to obtain formoses by this method, the synthesis gases obtained from the large scale industrial production of formaldehyde are fed continuously or intermittently at temperatures of between 10 and 150.degree. C. into an absorption liquid consisting of water, mono- or poly-hydric low molecular weight alcohols and/or higher molecular weight polyhydroxyl compounds and/or compounds capable of enediol formation as cocatalysts. The absorption liquid may contain soluble or insoluble metal compounds as catalysts (optionally bound to high molecular weight carriers) and has a pH of from 3 to 10. The formaldehyde is condensed in situ in the absorption liquid (or optionally in a reaction tube or cascade of stirrer vessels following the absorption liquid). The autocondensation of the formaldehyde is stopped by cooling and/or by inactivation of the catalyst with acids when the residual formaldehyde content in the reaction mixture has fallen to 0-10% by weight. The catalyst is finally removed. Further details of this process may be found in German Offenlegungsschriften Nos. 2,721,093 and 2,721,186.
Formoses prepared by this process may subsequently be converted into their hemiacetals with excess formaldehyde or they may be .alpha.methylolated by reacting them with formaldehyde in the presence of bases. Modified formoses of this kind have also been described in some detail in German Offenlegungsschrift No. 2,721,186 and are also included in the term "formose" in the context of the present invention.
The properties of the formoses (average hydroxyl functionality, degree of branching, proportion of reducing groups) can be varied within wide limits by controlling the reaction conditions of formaldehyde condensation. As a general rule, the further the stage to which the condensation reaction is continued (i.e. the lower the residual formaldehyde content when the condensation reaction is stopped) the higher is the average molecular weight and hence the hydroxyl functionality of the formoses obtained. If the condensation reaction is continued to a residual formaldehyde content of from 0 to 1.5% by weight, a formose which contains approximately 25% by weight of constituents with 5 carbon atoms, 45% by weight of compounds with 6 carbon atoms and approximately 20% by weight of compounds with 7 or more carbon atoms is obtained. At the same time, a total of only about 10% of polyols, hydroxyketones and hydroxyaldehydes having 2, 3 and 4 carbon atoms is obtained. This corresponds to an average hydroxyl functionality of about 5.
If formaldehyde autocondensation is stopped at somewhat higher residual formaldehyde contents, the distribution of components in the starter mixture obtained is different. When the condensation reaction is stopped at a formaldehyde content of from 2 to 2.5%, for example, a mixture of higher hydric alcohols, hydroxyaldehydes and hydroxyketones having an average hydroxyl functionality of approximately 4 is obtained. Other distributions of components, with much lower hydroxyl functionalities, are obtained when the condensation reaction is stopped at residual formaldehyde contents higher than 2.5.
The functionality of the products may be varied even further in the desired direction by mixing the formoses with difunctional or higher functional low molecular weight alcohols. Low molecular polyhydric alcohols with molecular weights up to about 300 suitable for this purpose include, for example, ethylene glycol, propane-1,2-diol, propane-1,3-diol, butane-1,4-diol, diethyleneglycol, dipropyleneglycol, triethylene glycol, tetraethyleneglycol, dibutyleneglycol, glycerol, trimethylolpropane, pentaerythritol, sorbitol, butanetriols and hexanetriols as well as ethoxylation products of these alcohols and hydrogenated formoses (formite). Amines and/or ethanolamines may also be used in the mixtures. Examples of such amines and ethanolamines include mono-, di- and triethanolamine, mono-, di and triisopropanolamine, N-alkanolamines such as N-methyldiethanolamine and N-ethyldiethanolamine as well as lower aliphatic monoamines and polyamines such as ethylamine, ethylene diamine, diethylenetriamine and triethylenetetramine.
According to an earlier proposal described in German Offenlegungsschriften Nos. 2,639,084, 2,714,084 and 2,714,104, formoses may be used as polyol components in the polyisocyanate polyaddition process for the production of polyurethane resins.